The invention relates to a method of cooling the friction surfaces of a wet interleaved friction disc by using vaned impellers acting as inward pumps. Commonly used cooling methods are forced flow outward circulation; or dip or splash cooling as the rotating plates dip into a sump or reservoir.
The use of forced flow cooling places a high emphasis on oil passage design including plate grooving, oil dispersion to the friction surfaces as well as rotating seals, a sump reservoir and a positive displacement pump. e.g. Aschauer U.S. Pat. No. 3,833,100 Higher cost and complexity result. In sump cooling the rotating discs may dip into a sump or impinge an annular surrounding oil reservoir or torus created by centrifugal action. If the plates dip into these reservoirs excessively, for example, one third dip to full immersion objectionable viscous drag occurs resulting in heating and power loss.
Considerable effort has been devoted exploring the optimum friction surface cooling, balancing excess oil flow with ensuing drag loss, versus insufficient cooling giving limited or marginal oil cooling amounts for certain discs in the pack. Speed of operation affects with internal outward forced cooling as well as dip or splash cooling.
At lower speeds centrifugal pressure is relatively low and circulation may be poor for forced cooling. Conversely at high speeds the coolant may be accelerated too rapidly and follow a path of least resistance resulting in incomplete friction surfaces cooling.
For dip or splash cooling the dip level, surrounding housing clearance and speed all affect performance. Higher speeds tend to fling coolant away from the discs depriving them of internal surface cooling.
Commonly used friction plates consist of an annular metal core with toothed drive means faced by annular friction discs on both sides bonded to the core.
The friction material compositions are well known as paper, sintered metals or graphitic with suitable coolant passage grooving, such as radial, angular, curved or waffle patterns, e.g. Aschauer U.S. Pat. No. 3,063,531. The friction surfaces operate against adjoining plates usually having a flat smooth steel surface. For the present invention vanes extending from the outer diameter of the friction plate cores act as radial flow pumps.
A radial flow pump conventionally consists of a rotating bladed impeller surrounded by stationary walls discharging into confined surrounding chamber. The performance is projected by the difference in inlet and outlet velocity diagrams which include both absolute rotational velocity and relative velocity between the moving fluid and moving impeller blade. Velocity is converted into pressure.
Pump design is a complex procedure relying heavily upon existing pump performance data. Consequently the determinations for the vaned impellers used in the present invention analysis are uncertain to the extent that substantial reserve factors are included in the analysis.